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In the design of high efficiency low pressure sodium lamps, six key parameters must be optimised to deliver the best performing lamps These are as follows:
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Discharge tube operating close to 260°C
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Excellent thermal insulation
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Low discharge current density
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Large surface area to volume ratio for discharge tube
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Low inert gas filling pressure
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Efficient electrode design
While each of these criteria is fairly easy to obtain by itself, there are some conflicts The most notable of these is the fact that both a low current density and a large surface area to volume ratio is called for.
Low current density calls for long discharge tubes of large diameter, thus explaining the large physical size of the low pressure sodium lamp. However, efficacy also rises for discharge tubes having a large surface area to volume ratio The reason for this is that sodium vapour is opaque to its own yellow radiation, i.e. it self-absorbs its own light If a large diameter discharge tube is employed the efficiency of light generation is very high, but such a source would be particularly useless because much of the light created at the core of the discharge tube would be absorbed before it could get out.
In practical lamp design, there is a cross-over point where the two conditions can be chosen to deliver a good compromise Increasing tube diameter from this value may create a more efficient discharge, but total efficacy falls due to self-absorption Decreasing tube diameter results in better light transmission, but light efficacy still falls because the discharge itself does not generate light so efficiently Low wattage sodium lamps operate at a discharge current of 0.6 Amps and for these ratings, a tube of approximately 12mm inside diameter has been chosen as the optimum High wattage lamps operate at 0.9 Amps or greater, and a 16mm diameter discharge tube is employed In the SOX-E range, extra efficacy is attained by operating the low watts lamps at 0.3A but in the same tube diameter, and the high watts at 0.6A, again in the same larger bore tube Although efficacy is increased drastically, luminous flux falls because the current loading in a tube of the same dimensions is lower.
The Linear Concept
In the 1950's, some very innovative thinking at BTH Mazda delivered a radical new concept in sodium discharge tube design - namely the employment of tubes having non-circular cross-sections By moving away from a circular tube to a design having a greater surface area to volume ratio, sodium light transmission can be considerably enhanced while also permitting the use of large diameter tubes which offer a low current density Many hundreds of discharge tube geometries were considered and tested, most of which were horrendously difficult to manufacture and were only ever laboratory samples.
By 1959, AEI Mazda had completed its development, and the Linear Sodium lamp was placed on the market The first product was a 200 watt lamp delivering 20,000 lumens, a very significant milestone in lighting history because this was the first commercially available light source to break the 100 lumen per watt barrier Other types rated 250W, 150W and 100W arrived shortly afterwards At the heart of these lamps was a discharge tube manufactured having an alternating crescent-shaped cross section which could be moulded into the discharge tube glass relatively easily - a cross-section being show in Figure 22 A set of moulds was depressed into one side of the heated glass tube, which was then rotated 180 degrees and the same moulds pressed into the opposite side of the glass to form the special dented shape The primary reason for grooving the tube on alternate sides was to ensure that the photometric light distribution of the lamp was symmetrical A valuable spin-off was that it increased the discharge length by approximately 10%, so more light could be generated from shorter lamps The concept for this shape of discharge tube originated from the PowerGroove linear fluorescent product, which was being developed by GE of America at the same time and the full co-operation that occurred between BTH and GE accounts for the similarity in design of many of their lamps and production techniques.
Figure 22 - Cross-section of the crescent-shaped discharge tube
In the 1960's, Osram-GEC also ventured into the manufacture of Linear Sodium, employing a design substantially based on the Mazda lamp German Osram also produced this lamp for a very brief period but withdrew it due to a lack of success selling this light source in their home market, in which white mercury light was strongly favoured So efficient was the SLI lamp that at this time, AEI Mazda decided to cease production of the old U-shaped design and concentrate all of its efforts into Linear Sodium technology This situation remained for more than ten years until the company was taken over by Thorn Lighting and a new SOX line was constructed
Heat Reflection in Linear Sodium
Linear sodium lamps were initially of the Integral design, in which the discharge tube was sealed into a one-piece evacuated outer bulb One or two glass sleeves surrounded the inner tube and served to minimise thermal radiation losses in just the same was as SOI Integral lamps The linear equivalents soon adopted their own name SLI to differentiate them as being linear products.
In 1965, Osram-GEC made a second major breakthrough in enhancing the efficacy of sodium lamps, and introduced the concept in their Linear range The company was successful in eliminating the glass heat conserving sleeves and replaced them with a thin film of pure gold sputtered onto the inside of the outer jacket which offered far better thermal insulation The design later evolved into two films side by side with clear stripes of glass between them, the uncoated portions being aligned with the sides of the discharge tube which radiated most light so as to maximise light transmission efficacy without seriously compromising heat insulation Osram's new products were marketed as the "Golden Linear" range for a number of years in sizes of 60W, 160W and 200W German Osram employed films of Bismuth which was not quite so efficient, and offered similar lamps of 175W and 220W whose length was marginally longer than the standard AEI and GEC style
In 1966 Philips launched U-shaped sodium lamps having a greatly improved coating consisting of tin oxide This semiconductor material offered excellent infra-red reflection but with minimal light absorption, and the SOX type IR-coated lamps were launched at this time SLI products were quick to adopt this new material, and the 60W, 160W and 200W lamps were relaunched with higher lumen outputs German Osram abandoned low pressure sodium production at this time.
Two years later, Philips developed the tin-doped indium oxide film which offered even greater efficacy, and SOX lamps were re-rated to lower wattages with the introduction of this coating The only SLI lamp to adopt the indium film was a new 140W product launched by Thorn Lighting, the successor of AEI Mazda The 140W SLI lamp was designed for operation on a 90W SOX ballast and had an incredible 125 lumen per watt efficacy The rest of the SLI range continued to be made with tin oxide coatings As with SOX lamps, tin and indium coated SLI products can be differentiated by the colour of the surface reflections in the glass - these are yellow coloured for tin lamps, and green for indium.
Second Generation Linear Sodium
When the SLI Linear Sodium lamp was launched, it marked a radical step forward from the old U-shaped SOI Integral format and offered considerably enhanced efficacy However by the mid 1970's, IR-coated SOX lamps were beginning to threaten the SLI market because they had overtaken SLI's groundbreaking efficacy and the new sleeveless SOX design offered a far more robust and compact format In the 1970's Thorn Lighting (the successor of AEI Mazda) invested heavily in SLI production and made a number of improvements in an attempt to fight off SOX, which the company did not manufacture at that time.
Most significant was the company's investment in automatic glassblowing lathes capable of forming the discharge tube into some of the very complex shapes which had been tested in the 1950's but were not economical to manufacture at that time Thorn's new lamps abandoned the simple crescent-shape grooved tube in favour of a lamp having a four-leaf clover style cross section, and this has a very large surface area to volume ratio Eighty-eight small ridges were moulded along the apex of the 'leaves', and these indents, affectionately known as greenhouses because of their shape, served as sodium retention reservoirs In 1980 a further gain in efficacy was made in which an even more complex 5-leaf clover tube was introduced with the new 200W High Output lamp Figure 23 shows a photograph of a sectioned piece of 5-leaf discharge tube.
Figure 23 - Cross-section of Thorn's 5-leaf Clover shaped discharge tube
Shortly after the time of the change in shape, the rare gas filling was also modified to optimise performance An addition of 0.05% Xenon permitted the argon concentration to be dropped and because xenon is not absorbed by the borate glass employed in sodium lamps, gas life was extended More significantly though, xenon allowed the total gas filling pressure to be reduced to very low levels A corresponding increase in efficacy was achieved, resulting from there being fewer elastic and inelastic collisions between the active discharge and the rare gas However this would have been accompanied by a decrease in lamp life had a new electrode design also not been introduced lower gasfill pressures result in an increased rate of evaporation of the emitter coating on the electrodes Figure 24 shows the difference in discharge colour of the rare gas in old and new style lamps The characteristic neon-red colour formerly employed is seen above, while the faint purple discharge below is that of the improved neon-argon-xenon mixture.
Figure 24 - New and old SLI gas fillings - Ne-Ar (above) and Ne-Ar-Xe (below)
Electrodes in sodium lamps have traditionally taken the form of triple coiled tungsten wire, dipped in emitter and with the emitter filling the spaces between the coils In this design, emitter is free to be mechanically shaken off through the powerful low-frequency vibrations found in streetlighting service, or it can be sputtered and evaporated away during normal lamp operation In a spinoff from the company's work on improved cathodes for fluorescent tubes, an entirely new type of sodium electrode was developed, known as the Braided Cathode Making use of a machine borrowed from the textiles industry, seven fine wires of tungsten were braided into a hollow tube The tube itself was then coiled to form the electrode At the emitter impregnation stage, this chemical was drawn right inside the braided tube and each cathode of a given size and rating could hold a far greater amount of emitter Thus lamp life could be enhanced tremendously The braided cathode resulted in the company doubling the life of its entire fluorescent lamp range immediately, and its application to Linear Sodium technology helped make the new clover-leaf style lamps more long lived than ever before Figure 25 illustrates a highly magnified view of the braided cathode.
Figure 25 - Thorn Lighting's Innovative Braided Cathode
Aside from being the first practical lamp to pass the 100lm/W mark, Linear Sodium also holds another record in lighting history It was the first light source which was put to use in motorway illumination - until 1971 only inner city streets and main trunk roads had been illuminated In this year however, the first motorway lighting installation was commisioned, consisting of 1200 of the Thorn 140W SLI/H lamps on the M1 motorway between London and Luton The lamps were operated in the AEI's Amberline SLI lantern, which later became better known as Thorn Lighting's Alpha Five luminaire.
Before concluding the section on Linear Sodium, it is of interest to note that Osram-GEC made many experiments at its Hirst Research Centre in Wembley on High Pressure Sodium lamps having similarly shaped ceramic arc tubes of non-circular section Rather than enhancing the discharge efficacy, the primary goal here was to deliver an asymmetric radiation of light to better suit streetlighting optics Some designs were very complex and even had lenses and fresnel patterns moulded into the ceramic discharge tube itself Despite very encouraging results, the production costs were prohibitively high at the time Full details are disclosed in S.A. Rigden's US Patent 3,885,181 of 1975 which can be viewed at www.uspto.gov
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